This invention relates to secondary radar systems, that is to say systems in which distances are measured by means of radiating a signal from a measuring station, which receives a return signal from an active target or reflecting station.
It is known to employ a pulse radar to interrogate an active target which, on reception of a pulse from the measuring station, transmits a pulse or, more usually, a coded reply after a predetermined time. Usually the measuring station is arranged to compute or display the range of the target. The present invention, however, employs continuous wave transmission and an indication of range is obtained by comparison of the phase of modulation of the reflected wave with the phase of similar modulation of the transmitted wave. The time of propagation of a wave retransmitted from a different reflecting object is, approximately, 1 ms per 150 Km and accordingly unambiguous measurement of the distance of the reflective object can be made by a comparison of the phases of the respective modulations provided that the frequency of transmission in kHz is less than 150 divided by the distance in kilometers. It is intended that the present invention should be used in combination with radio telephones or other forms of mobile transceivers. Accordingly, it is intended that a receiving station as described hereinafter should be part of a mobile transceiver and is operative to transmit or receive according to the state of the transmit/receive switch in the mobile transceiver. The modulation which is preferably employed in this invention is at an audio frequency and accordingly the various modulating, demodulating and amplifying circuits that would be present in a mobile receiver may be employed for the performance of the present invention. The measuring station may be similarly adapted for use in or as part of a mobile receiver, it being thereby possible for the operators of two portable or mobile transceivers to measure the distance separating them.